1. Field of the Invention
The present invention relates to an LED driving circuit for driving an LED, and an LED illumination appliance, an LED illuminator, and an LED illumination system, each using an LED as a light source.
2. Description of Related Art
An LED has features of low power consumption and long lifetime and has a spreading use not only for display devices but also for illumination equipment. For LED illumination appliances, a plurality of LEDs is used in many cases so as to obtain a desired illuminance.
General illumination equipment uses a commercial AC power source (AC 100 V in Japan) in many cases. In view of the case where an illumination appliance using the LED (hereinafter, referred to as “LED illumination appliance”) is used in place of a general illumination appliance such as an incandescent bulb, the LED illumination appliance is desired to have a configuration using an AC power source as in the case of the general illumination appliance.
Moreover, in an illumination system for performing dimming control on the incandescent bulb, a phase-control type dimmer (generally referred to as “incandescent light controller”) is used. The phase-control type dimmer enables easy dimming control by controlling power supply to the incandescent bulb with a single volume control element when a switching element is turned ON (in general, a thyristor or a triac element) at a certain phase angle of an AC power supply voltage.
For dimming control of the LED illumination appliance using the AC power source, it is desirable to use the phase-control type dimmer as in the case of the dimming control of the incandescent bulb. Here, a conventional example of the LED illumination system capable of performing the dimming control on the LED illumination appliance using the AC power source is illustrated in FIG. 13.
An LED illumination system illustrated in FIG. 13 includes a phase-control type dimmer 2, an LED driving circuit 94, and an LED module 3. The LED driving circuit 94 includes a diode bridge DB1, an impedance adjusting section 96, and an LED current control circuit 5. The phase-control type dimmer 2 is connected in series between an AC power source 1 and the LED driving circuit 94. The phase-control type dimmer 2 receives an AC voltage from the AC power source 1 to determine a phase at which a triac Tri1 corresponding to a current holding element is to be turned ON, by resistors RV1 and RV2, capacitors CV1 and CV2, and a diac Di1. Besides the triac, a MOS switch or the like is used as the current holding element in some cases.
The phase-control type dimmer 2 includes a noise-eliminating circuit including a capacitor CLC and an inductor LLC. In the phase-control type dimmer 2, the capacitor CLC determines an impedance when the triac Tri1 is not in an OFF state. The inductor LLC prevents a short-circuit current from flowing through the capacitor CLC when the triac Tri1 is turned ON.
For the dimming of the incandescent bulb by the phase-control type dimmer, it is known that the dimming cannot be normally performed when an incandescent bulb having a small wattage is connected to the dimmer because flickering or flashing occurs. The LED has a remarkably small wattage as compared with the incandescent bulb. Therefore, the flickering or flashing becomes a problem also in the case of the dimming of the LED with the phase-control type dimmer. Thus, in order to prevent the flickering or flashing in the LED illumination system described above, the impedance adjusting section 96 is used. Examples of the impedance adjusting section 96 include an OFF-time impedance adjusting section 961, an impedance adjusting section 962 for preventing malfunction due to resonance, and an impedance adjusting section 963 for maintaining a triac current. Hereinafter, each of the impedance adjusting sections is described referring to the drawings.
In the LED illumination system, when the phase-control type dimmer 2 is in an OFF state, that is, the triac Tri1 is in the OFF state, normally, power supply from the AC power source 1 to the LED driving circuit 94 should be cut off. However, the AC power source 1 and the LED driving circuit 94 are constantly electrically connected to each other by the capacitor CLC included in the noise-eliminating circuit of the phase-control type dimmer 2. Therefore, a finite impedance is generated by the capacitor CLC in the LED driving circuit 94 even when the triac Tri1 is in the OFF state. When the impedance is low, the phase-control type dimmer 2 does not operate normally in some cases. Therefore, the OFF-time impedance adjusting section 961 performs adjustment so that the impedance of the LED driving circuit 94 becomes sufficiently lower than that of the capacitor CLC. In this manner, a voltage is applied to both terminals of the phase-control type dimmer 2 when the phase-control type dimmer 2 is in the OFF state, and hence the phase-control type dimmer 2 operates normally. Note that, the OFF-time impedance adjusting section 961 is in an OFF state when the phase-control type dimmer 2 is in an ON state.
A circuit diagram illustrating an example of the OFF-time impedance adjusting circuit 961 is illustrated in FIG. 14A. As illustrated in FIG. 14A, the OFF-time impedance adjusting circuit 961 uses a comparator EA to compare a driver voltage applied to the LED driving circuit 94 and a voltage of a voltage source VA. The OFF-time impedance adjusting circuit 961 turns ON a switching element MA (MOS) when the driver voltage is lower than the voltage of the voltage source VA.
When the phase-control type dimmer 2 is in the OFF state, the driver voltage of the LED driving circuit becomes lower than the voltage of the voltage source VA. Therefore, a HIGH signal is input from the comparator EA to the switching element MA (in other words, a signal for impedance adjustment is input). As a result, the switching element MA is turned ON, and the impedance of the OFF-time impedance adjusting section 961 becomes an impedance determined by a resistance value of a resistor RA3. On the other hand, when the phase-control type dimmer 2 is in the ON state, the driver voltage becomes higher than the voltage of the voltage source VA. Therefore, a LOW signal is input from the comparator EA to the switching element MA (in other words, the signal for impedance adjustment is not input). As a result, the switching element MA is turned OFF, and the impedance of the OFF-time impedance adjusting section 961 becomes high.
Next, the impedance adjusting section 962 for preventing malfunction due to resonance is described. When the phase-control type dimmer 2 is turned ON, a current flowing through the triac Tri1 oscillates due to a resonance phenomenon between the capacitor CLC and the inductor LLC of the noise-eliminating circuit, which sometimes results in turning-OFF of the triac Tri1. Therefore, the current is controlled to flow through the impedance adjusting section 962 for preventing malfunction due to resonance so as to release energy stored in the capacitor CLC while the phase-control type dimmer 2 is in the OFF state, thereby suppressing the resonance between the capacitor CLC and the inductor LLC. Note that, the impedance adjusting section 962 for preventing malfunction due to resonance is required to temporarily allow a large current (for example, 200 mA for 200 μs) to flow therethrough, and therefore can be configured to include the capacitor or a combination of the capacitor and the resistor.
A circuit diagram illustrating an example of the impedance adjusting section 962 for preventing malfunction due to resonance is illustrated in FIG. 14B. As illustrated in FIG. 14B, the impedance adjusting section 962 for preventing malfunction due to resonance uses a comparator EB to compare the driver voltage applied to the LED driving circuit 94 and a voltage of a voltage source VB. When the driver voltage is higher than the voltage of the voltage source VB, a HIGH signal is output from the comparator EB. When the HIGH signal is output from the comparator EB, a switching element MB is turned ON.
While the switching element MB is in an ON state, a current flows through a current source IB to lower the impedance of the impedance adjusting section 962 for preventing malfunction due to resonance. The switching element MB is turned OFF depending on a time constant determined by an electrostatic capacitance of a capacitor CB and a resistance value of a resistor RB4. When the switching element MB is turned OFF, the current does not flow through the current source IB. As a result, the impedance of the impedance adjusting section 962 for preventing malfunction due to resonance becomes high.
As the last impedance adjusting section, the impedance adjusting section 963 for maintaining a triac current is described. The power consumed by the LED module 3 is small. That is, an amount of a current flowing through the LED module 3 is small, and an amount of a current flowing through the phase-control type dimmer 2 is correspondingly small. In view of this fact, the current flowing through the triac Tri1 sometimes becomes lower than a holding current. In such a case, a malfunction in which the triac Tri1 is turned OFF sometimes occurs. Therefore, the impedance adjusting section 963 for maintaining a triac current lowers the impedance when the amount of the current flowing through the triac Tri1 is small so as to allow the current to flow therethrough. In this manner, the current flowing through the triac Tri1 is maintained to be equal to or higher than the holding current.
A circuit diagram illustrating an example of the impedance adjusting section 963 for maintaining a triac current is illustrated in FIG. 14C. As illustrated in FIG. 14C, the impedance adjusting section 963 for maintaining a triac current is configured to use the resistor RA of the OFF-time impedance adjusting section 961 as a current source IC, and a basic operation thereof is the same as that of the OFF-time impedance adjusting section 961. The impedance adjusting section 963 for maintaining a triac current compares the driver voltage and a voltage of a voltage source VC. Depending on the result of the comparison, a current flows through the current source IC to lower the impedance of the impedance adjusting section 963 for maintaining a triac current.
As described above, some of the impedance adjusting sections use the resistor and the switch (the switch may be any of a mechanically driven switch and a semiconductor switch such as a MOS), others use a semiconductor active element such as a MOS or a transistor, which performs constant-current control. Note that, the impedance adjusting section is sometimes referred to as a current drawing section because the current flows therethrough. Specifically, the flow of the current through the impedance adjusting section allows a stable operation of the phase-control type dimmer 2 (dimming of the LED illumination appliance to be normally performed). Note that, as the impedance adjusting section, one of the three types of the impedance adjusting sections described above is used. In some cases, two or more types of the impedance adjusting sections are connected in parallel.
In an illumination system, pluralities of illumination appliances (LED illumination appliances) are often connected to the single phase-control type dimmer 2. In the illumination system described above, the pluralities of illumination appliances are simultaneously used in some cases. In other cases, however, each of the illumination appliances is used alone. For stable lighting of the illumination appliance when the illumination appliance is used alone, each of the illumination appliances includes the impedance adjusting section. The impedance adjusting section is configured so as to be able to sufficiently operate alone.
In the illumination system described above, a current required for the stable operation of the phase-control type dimmer flows through the impedance adjusting section when the illumination appliance is used alone. When the pluralities of illumination appliances are simultaneously used in the illumination system configured as described above, the current required for the dimming flows through the impedance adjusting section included in each of the illumination appliances.
The current flowing through the impedance adjusting section is a current which is not used to light the illumination appliance (LED), that is, a loss current. In the illumination system described above, when the pluralities of illumination appliances are simultaneously used, the loss current flows through the impedance adjusting section of each of the illumination appliances. Therefore, loss power becomes correspondingly large.